Cutaneous burns are one of the most destructive insults to the skin, causing damage, scarring and even death of the tissue. It has been reported that burns alone account for over 2 million medical procedures every year in the United States. Of these, 150,000 refer to individuals who are hospitalized and as many as 10,000 die (Bronzino, 1995, The Biomedical Engineering Handbook (CRC Press: Florida)). Despite the large number of annual burn cases, the accurate assessment of burn severity remains a problem for the burn specialist. The ability to distinguish between burns that will heal on their own versus those that will require surgical intervention is particularly challenging. Generally, the depth of a burn injury determines and is inversely related to the ability of the skin to restore and regenerate itself. Burns involve damage to the dermis in varying amounts, reducing the dermal blood supply and altering the skin hemodynamics. Highly destructive burns have only a marginal residual blood supply to the dermis that may result in ischemia and ultimately necrosis of the dermis, as the re-epithelialization of the tissue depends on the viable dermis below the burned tissue. Thermal injuries are clinically classified according to the depth of the injury as superficial (epidermal), partial thickness (epidermal and varying levels of dermal) and full thickness (epidermal and dermal). Superficial burns are mild burns whereby the tissue is capable of regenerating the epidermis. Partial thickness injuries destroy a portion of the dermal layer, although sufficient dermis usually remains for re-epithalization to occur with adequate vasculature. Deep partial and full thickness injuries involve destruction of the dermal layer and what little if any remains of the dermis is insufficient to regenerate due to a reduced dermal blood supply. Currently, the diagnosis is usually done by visual inspection and is based on the surface appearance of the wound.
As a research tool, biopsies followed by histological examination remain the gold standard for gauging burn depth (Chvapil et al, 1984, Plast Reconstr Surg 73: 438-441). However, the major drawback of this technique is that it provides a static picture of the injury reflecting the extend of tissue damage at the time the biopsy was taken. Since burn injuries are dynamic and change over the early postburn period, a single biopsy taken at the initial assessment of the injury may not accurately predict outcome. For this reason, biopsies are not generally relied upon in the clinical assessment of burn injuries.
Fluorescent dyes, such as indocyanine green, have also been used to assess the severity of burns. This invasive method, which is used specifically to monitor tissue perfusion, requires that a fluorescent dye be injected into the systemic circulation of a patient (Gatti et al, 1983, J. Trauma 23: 202-206). Following the injection of dye, vessels that are intact and have a functional blood supply will fluoresce when illuminated by the appropriate wavelength of light. The presence or absence of dye fluorescence therefore acts as an indicator of tissue perfusion. While this method has demonstrated success in distinguishing superficial from full thickness burns (i.e. presence or absence of fluorescence), it cannot easily differentiate those burn types that are between the two extremes. Furthermore, the extended washout times of the dye limit the frequency with which it can be used to assess a dynamic injury. As a result, indocyanine green has not yet met with clinical acceptance even though it has been available for burn diagnosis for over a decade. Other techniques, including thermography (Mason et al, 1981, Burns 7: 197-202), laser Doppler (Park et al, 1998, Plast Reconstr Surg 101: 1516-1523), ultrasound (Brink et al, 1986, Invest Radiol 21: 645-651) and light reflectance (Afromowitz et al, 1987, IEEE Trans Biomed Eng BME34: 114-127) have also been developed to assess burn injuries.
U.S. Pat. No. 5,701,902 describes the use of fluorescence excitation and simultaneous IR spectroscopy to characterize burns. Specifically, in this method, the fluorescence of elastin, collagen, NADH and FAD are analyzed, and the total amount of hemoglobin and relative amounts of oxygenated hemoglobin and reduced hemoglobin as well as the water reflectance are also determined. The data is then compared to data from similar skin types in a database which is in turn used to characterize the burn. As can be seen, this process is invasive as it requires the injection of fluorescent dyes and also relies on the use of a database for characterizing the burn injury.
U.S. Pat. No. 4,170,987 teaches a medicinal skin diagnosis method which uses a rotating mirror and three detectors (IR, red and green) onto which the same pixels of the patient's skin sampled in the line scan are simultaneously imaged. From the respective three associated stored digital values per pixel, ratio numbers are then formed which can be displayed on a color monitor as a false-color image or can be printed.
Canadian Patent Application 2,287,687 teaches a device for generating data for the diagnosis of the degree of injury to a patient's skin tissue wherein a halogen lamp is used to illuminate a skin portion. The remitted light from the skin surface is recorded by a multispectral camera and the spectral images are analyzed pixelwise using suitable software. Classification of the skin injury is carried out by specific ratio formations and comparison values of degrees of injury to known skin tissue patterns.
As discussed above, the most widely used diagnostic method for diagnosing burn injuries remains visual evaluation by an experienced physician. The prior art methods described above either provide a static picture of a burn injury or rely on databases for assistance in diagnosing burn injuries. Clearly, the need exists for a reliable, non-subjective, and easy to handle technique to evaluate burn injuries in the early post-burn period that provides diagnostic as well as prognostic information on the severity of the injury.